Alligator Gar As Means To Control Asian Carp


        Jordan Fielder, a nineteen year old boy, was enjoying a fun day on the Illinois river with his family when all of a sudden a large fish launched from the water like a missile, and smashed into his face. The fish fractured his nose, dented his forehead, and shattered bones in his eye sockets and brow (Schankman 2015). Jordan commented, “If it had hit me any harder it could have broken my skull bones and essentially damaged my brain and killed me on the spot”(Schankman 2015). For Jordan this was a fun family day on the river, turned to a near death experience. The fish responsible for this, is the invasive Asian carp, which is overrunning the Illinois river and its surrounding waters including the Mississippi, Missouri and Ohio rivers (Hayer et al. 2014). The carp easily become scared by boat motors or other loud noises which causes them to jump out of the water (Shankman 2015), turning their large bodies into a dangerous projectile which can clearly hurt people in their path. Jordan’s experience is not uncommon as this has happened to many others. As harmful as they can prove to humans, they are just as bad for the ecosystem, as is seen with many invasive species.

        In the 1950s, East Africans introduced Nile perch to Lake Victoria to strengthen a lacking fishing industry (Micalizio, 2015, p. 1). The environmental effects of this introduction completely transformed the Lake Victoria ecosystem. Voracious appetites and bountiful prey allowed the top-tier predator Nile perch to cause the extinctions of over 200-species of fish native only to Lake Victoria, such as cichlids. Native predatory catfish species such as the Sudan catfish and African sharptooth catfish also suffered as a result of the Nile perch (Frans Witte, 1997). In 1973, the Sudan catfish and African sharptooth had catch rates of 44 pounds per hour. However, by 1985 they had catch rates of zero. Whereas the Nile perch showed a catch rate of zero in 1973, but jumped to 176 pounds per hour by 1987, 4 times higher than the native catfish species when they were at peak abundance (p. 28). A domino-effect occurred from the loss of native species, leading to outbreaks of insects and algal blooms (Nile perch , 2014, p. 6). Some of the largest impacts came down on the backs of humans. The fishermen and their families cannot eat the fish themselves, they have too high a value and eating them means a loss in profits, yet fishermen go on longer fishing trips now than in the past to try and keep up with demand (p. 9). The Nile perch serve as an example that represents how non-native freshwater fish introductions can derail an ecosystem and community if not well-controlled or managed (Vitule et al., 2009). There is such an introduction happening right under our noses here in the United States, the invasive Asian carp.

        Ecosystems have a delicate balance in which organisms work in harmony, each occupying their own little niche (the role of an organism in an ecosystem) (, 2016), when a new organism enters that ecosystem, they can occupy another species’ niche, competing with them for resources and food. Unfortunately, native species often lose this competition to the new invaders. The National Invasive Species Information Center (NISIC), defines invasive species as “non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health” (“What is an invasive species?” 2012, para. 1). One such invasive species, the Asian carp, have made a name for themselves here in the United States, introduced to control phytoplankton and for aquaculture.

        Native species of carp have existed in the United States for over 100 years, and the species called the “common carp” has lived here with little environmental impact (Naylor et al., 2001).  The newer and more potent Asian carp describe 4-different species, the bighead, grass, black, and silver carp. The U.S. imported silver and bighead carp in the 1970s from Asia for research purposes, putting them into wastewater lagoons and aquaculture ponds and observing if they improved water quality (Naylor et al., 2001, para. 2). Federal and state agencies, private citizens, and researchers imported and introduced the grass carp from eastern Asia in 1963 to control aquatic plants in fish farms (Grass carp, 2013, para. 1). Juvenile black carp came to the U.S., initially in Arkansas, in the 1970s when they arrived with a shipment of grass carp (Nico and Nielson, 2014). Nobody noticed because juvenile black and grass carp have nearly identical appearances. The U.S. attempted to use the black carp as a food resource and to control yellow grubs in aquaculture ponds (para. 5). Flooding events in aquaculture ponds connected to rivers allowed the silver, bighead, grass, and black carp to escape into the Mississippi River and Missouri River where they now have established breeding populations (Naylor et al., 2001, para. 2; Grass carp, 2013, para. 1; Nico and Nielson, 2014, para. 5).

        Invasive Asian carp demonstrate trends of rapidly increasing abundance a short time after their introduction. In the Missouri River in South Dakota, the abundance of Asian carp skyrocketed from 2009-2012 (Hayer et al., 2014, p. 294). In 2009, a fishing survey in the Missouri River caught no Asian carp. By 2012, fishing surveys caught 35 fish per hour (p. 294). In 6 sections of the Mississippi River, the number of Asian carp caught went from <50 per hour in 2003, to 775 per hour in 2012 after their introduction (Phelps et al., 2017, p. 7).  This 15x increase demonstrates the ability of Asian carp to overwhelm an area in as little as 9 years.

        Asian carp often outcompete native species for food (Asian carp overview, 2015). Asian carp filter feed and voraciously consume algae and zooplankton, primary food sources for native fish species like gizzard shad, paddlefish, and bigmouth buffalo (Asian carp overview, 2015; Irons et al., 2007; Sampson et al., 2009). Small zooplankton such as rotifers compose a large part of the diet of many native filter feeders, however, Asian carp consume them as well. In one section of the Mississippi River, Asian carp cut the abundance of rotifers from 6000 per liter of water in 2002, to 3500 in 2003, nearly a 50% decrease in only one year (Sampson et al., 2009, p. 488). Thus reducing the amount of available prey, and forcing predatory species to feast more heavily on other organisms such as copepods, seldom consumed by many fish, but compose nearly 62% of the diet of endangered paddlefish (p. 489). The decrease in rotifer abundance observed by Sampson et al. (2009) therefore means that fish will have to search for and eat different prey species instead of relying on rotifers the way they did before Asian carp.

        Asian carp reduce the abundance of native species where they colonize (Hayer et al., 2014; Phelps et al., 2017). In 2009, Asian carp represented <1% of the catch in the Missouri River, whereas the native emerald shiner fish comprised roughly 30% of fish caught in 2009. In 2012, Asian carp composed 50% of the catch, and emerald shiner dropped to 5% of the catch, equating to a 6x decrease in emerald shiner, and a 50x increase in Asian carp  (Hayer et al., 2014, p.298). In the Mississippi River, Asian carp caused the bigmouth buffalo population to decrease by 10%, instead of following the historically-observed increase of 35%. After the invasion of Asian carp, the number of buffalo caught per hour decreased from 178 to 85 (Solomon et al., 2016, p.8). In another study on the Illinois River, the bigmouth buffalo’s abundance declined by 80% in 2005, compared to the abundance recorded from fishing trips in 1995 (Irons et al., 2007, p. 268). In this same stretch of river, the annual Asian carp catch increased from 0 in 1995, to 500 in 2005 (p. 265)

        Predatory and game fish populations also undergo negative changes because before the young become large enough to eat other fish and crustaceans, they eat small plankton consumed by the invasive carp (Solomon et al., 2016, p.1). This means that if the carp kill off the young of a species, they will do massive damage to the species populations as a whole. For example, two species of crappie showed dramatic decreases in abundance, Black crappie populations decreased by 61.79% and white crappie populations decreased by 45.98% (Solomon et al., 2016, p. 8). Carp do not feed on crappie, but they feed on the same thing as the juveniles, causing the population to have trouble growing. The removal of plankton by Asian carp also casts residual effects on important prey species for predatory fish. For example Asian carp negatively affect gizzard shad, another filter feeder. These shad comprise an important food source for predators of the ecosystem (Phelps et al., 2017, p. 11). Shad are a staple food source of a very popular gamefish in the Largemouth Bass, if there are less shad, then the bass will not do as well (Storck et al. 2011, pg. 1). Gizzard shad went from an average biomass increase of 10% to nothing because the Asian carp reduced their survival rate from 80% to 10%, preventing their population from growing (Phelps et al., 2017, p.5). Fish catches also decreased by almost half for the shad going from 7186 per hour to 3810 per hour (Phelps et al., 2017, p.6). This massive decrease sends a negative effect right up the food chain of an ecosystem. Directly related to the shad population going down, the CPUE of Asian carp increased over the same period of time showing that the native fish get outcompeted (Phelps et al. 2017, p. 11). As carp became more prevalent in floodplain lakes, predators such as bass, catfish, gar and bowfin started to disappear. (Phelps et al. 2017, p.9).  

        One reason that carp have become so abundant is that native fish have shown a preference for native prey when given a choice between the two. Native piscivores of the Mississippi River Basin showed negative selectivity or preference of silver carp versus native prey species (Wolf et al. 2017, p. 1142). White bass tested in this study, chose Asian carp first only 3 of 29 times (Wolf et al., 2017, p. 1141). The study showed that largemouth bass chose to eat Asian carp first instead of native prey species only 4 of 29 times (Wolf et al., 2017, p. 1141). However largemouth bass did show a positive selection of 0.23 specifically for grass carp, however they still negatively selected for Asian carp in general with a -0.08 (Wolf et al., 2017, p. 1141).  In this study a score of 1 represented the highest selectivity for consuming Asian carp and a score of-1 represented a complete avoidance of Asian carp.  The study showed that all native piscivores showed little or no preference for Asian carp except the longnose gar, which had a selection for Asian carp of 0.12 (Wolf et al., 2017, p. 1141). Asian carp’s low selectivity by U.S. piscivores, (Wolf et al., 2017) demonstrates that using predators to control Asian carp infestations in U.S. waters will only be successful through the implementation of one of the carp’s natural predators into their new environments in the U.S.

        However as none of Asian carp’s natural predators live in U.S. water systems, all of Asian carp’s natural predators would also be invasive species to these ecosystems and their implementation into U.S waterways could cause further ecological impacts that are just as bad or worse than the negative impacts ensued by Asian carp infestations (National Wildlife Federation, n.d.). A situation similar to this occurred when the cane toad was introduced to Australia in an attempt to control pests. These toads succeeded at their job but caused many negative side-effects to the environment such as consuming large quantities of non-pest animals such as small lizards. (Frontier Gap, 2015, para. 4) These toads were able to grow in numbers and cause such havoc due to their toxic skin and glands which leave them with no predators in this new environment. (para. 4) With this knowledge at hand, the introduction of non-native predators in efforts to control the effects of Asian carp infestations does not seem like a  smart option.

        Invasive species cause vast amounts of damage to humans every year. The most recent economic study shows that the United States spends more than $120 billion every year to control invasive species (Scully, 2016, para. 3). In 2010 alone, the U.S. spent $78.5 million dollars to keep Asian carp from reaching the Great Lakes (“The cost of invasive species,” 2012, para. 11). That’s enough money to buy 20-Hubble telescopes every year (Goldman, 2012, para. 2). Even if you don’t care about fish, you should find this alarming because Asian carp affect rivers that flow in and out of the great lakes.With sixty-five million pounds of fish harvested from the great lakes every year, the lakes generate about one billion dollars in revenue for the local economy (“About our lakes: economy,” n.d., pg. 1). If Asian carp decrease native fish populations by eve one percent, that is a lot of money to lose. So clearly there is high potential for a significant problem to occur.

        So it is clear that asian carp cause problems wherever they invade. People are not blind to this and have done things to try to combat their invasion. The Army Corps of Engineers implemented an electric fence along the Chicago ship canals to keep them from moving upriver (Kraft, 2013, para 6) This is bad for two reasons. First, it stops native fish from moving along the river, and second the power for the fence has shut off, and carp moved past it (Kraft, 2013, para 6). Another option is dumping poison into the rivers to kill off the carp (Hasler, 2010, para 10). This is bad because it could kill off all of the native species along with it, and dumping poison into a river will only carry it further upstream, affecting more than just the target area. Furthermore, sometimes the poison just does not work (Hasler, 2010, para 5).  With people struggling to come up with a viable solution, we have a proposition; add a predator into to U.S. waterways to combat the Asian carp.

        Luckily there is one species of predatory fish native to the Southeastern U.S., called an alligator gar, that many scientists are arguing could be used as an effective predator of Asian carp. (“How to combat Asian carp? Get an alligator gar,” 2016). The use of a native predator could prevent against any negative effects that could be incurred from the of introduction of another invasive species to U.S. waterways. Alligator gar once existed through the Mississippi River and its tributaries all the way from Ohio to Illinois and down to the Gulf of Mexico. They now however, only live in in the Mississippi River valley from Arkansas southward (U.S. Fish and Wildlife Services, n.d., para. 12). The reason for this mass decline in alligator gar populations is mainly caused by humans. For one many people saw  alligator gars as a “trash fish” with less value than commercial game-fish and targeted them for extermination and control (para. 13). Some of the other main reasons that humans targeted alligator gar in this way include that they are big, monster looking fish, thought to attack humans and they were thought to deplete populations of commercial gamefish (Cermele, 2016). Although these two notions about alligator gars fueled the drive to eradicate these species, both of them ended up being false. There has never been a confirmed attack of an alligator gar on a human to date (Cermele, 2016, para. 6; Parks, 2016, para. 1). Additionally, alligator gar do not eat many game-fish as they are opportunistic feeders, eating anything that swims in reach of them, and most game fish are relatively stationary, meaning that if alligator gar wanted to eat them, they’d have to hunt them down, something that is just not in their nature (Cermele, 2016, para. 10; Department of Natural Resources, para. 10). Scientists have only disproven these false notion recently through studies allowing light to shine onto alligator gars potential for controlling Asian carp infestations and reintroduction efforts are already underway in Illinois (Department of Natural Resources, n.d., para. 5). State officials must consider two constraints to determine if reintroduction efforts of alligator gar in U.S. waterways will be an effective measure of combatting Asian carp infestations; effects of alligator gar on Asian carp populations and feasibility of an alligator gar reintroduction program.

        Obviously before considering feasibility of an alligator gar reintroduction, policy makers should determine the effects an alligator gar reintroduction will have on Asian carp populations in U.S. waterways. Many people including Dan Stephenson, biologist and chief of fisheries at the Illinois Department of Natural Resources, criticize that alligator gar can actually consume Asian carp, saying they aren’t big enough to do so. He says that their jaws just won’t open wide enough to fit most Asian carp (Garcia, 2016, para. 6). However alligator gar one of the  largest freshwater fish in North America and the largest fish species in the Mississippi River Valley (U.S. Fish and Wildlife Service, 2015, para. 3), an ecosystem that the Asian carp have spread throughout. At maturity they can grow to be 10 feet in length and weigh up to 300 pounds (“Alligator gar,” 2009, para. 2). On the other hand, the four Asian carp species that have invaded U.S. waterways can only grow to be at maximum, about 3.3-6 feet in length and weigh 70-99 pounds in weight (“Asian carp,” 2017). Considering that the biggest alligator gar can grow to be 3 times the weight of the biggest Asian carps in U.S. waterway and are opportunistic predators (Department of Natural Resources, n.d.), it is perfectly reasonable to assume that alligator gar can consume Asian carp, if not as full grown fish but at the very least as adolescents; which could be even more effective as it would reduce the amount of Asian carp surviving to reproductive maturity.

        Alligator gar do in fact mostly target rough fish, including carp, and gizzard shad (Cermele, 2016, para. 10, Department of Natural Resources, para. 10). Although data is scarce on alligator gar selectivity towards Asian carp specifically and is not well documented, more is known about other species of gars’ selectivity for Asian carp. According to recent research from from Western Illinois University the shortnose gar has a positive selection for Asian carp as they existed in the highest abundance, above any other prey item, in shortnose gars stomachs (David et al., 2016,  para. 5). Additionally, as previously stated, Wolf et al. (2017) found that longnose gar showed a positive selection for asian carp. Since these species are very closely related to alligator gar, it is likely alligator gar would have a similar, positive selection for Asian carp as longnose and shortnose gar and consume Asian carp in similar numbers. Alligator gar would likely even consume more Asian carp biomass per fish than longnose and shortnose gar, as they are the largest of the seven known gar species (Alligator gar et al., 2009, para. 2), adding to their effectiveness.

        With alligator gars effectiveness of controlling Asian carp infestations demonstrated, the next thing to consider is the feasibility of an alligator gar reintroduction program in U.S. waterways. Reintroduction programs have been implemented successfully in the United States on many occasions bringing animals such as California condors and black-footed ferret populations back from the brink of extinction (Errick et al., 2015). In 1982, less than 22 California condors remained. However, through reintroduction efforts by the U.S. Fish and Wildlife Service started in 1985, by 2015 there were about 210 of them in the wild and 180 in captivity (Errick et al., 2015, para. 8).

         On top of returning decimated species back to stable populations, predator reintroduction programs are also a tried and proven technique for combatting the effects of rampant species population growth. The reintroduction of gray wolves to Yellowstone National Park in 1995 had immense success combatting the effects of unwanted elk population growth. All the gray wolves of Yellowstone had been hunted to extinction by the end of the 1920s (“1995 Reintroduction of wolves in Yellowstone,” 2017, para. 4). Thereby allowing elk populations to skyrocket and mass degradation of brush and trees that elk graze on (Wolf reintroduction changes ecosystem, 2011, para. 8, 1995 Reintroduction of wolves in Yellowstone, 2017, para. 5). However, in the winter of 1995/1996, scientists captured 14 gray wolves from Canada and released them into Yellowstone Park (“Wolf reintroduction to Yellowstone Park, wolf pack dynamics, & wolf identification,” 2000, para. 2). By 2015, there were about 528 total wolves in the Greater Yellowstone ecosystem (“Wolves,” n.d., para. 6). Soon after their reintroduction into Yellowstone the environment started to return to a healthy state. This increase in ecosystem health isn’t just because the wolves ate the elk and drove their populations down. In fact elk populations have actually increased since Gray wolf reintroductions into Yellowstone. For instance in 1968, only about one-third of today’s  elk numbers existed in Yellowstone (“Wolf reintroduction changes ecosystem,” 2011, para 9). Willow tree health in Yellowstone also increased (“1995 Reintroduction of wolves in Yellowstone,” 2017; “Wolf reintroduction changes ecosystem,” 2011). If 108 gray wolves living in Yellowstone can have these positive effects on the ecosystem by consuming an increasing population of elk, it is likely that a small population of alligator gar, another top predator (U.S. Fish and Wildlife Service, 2015), can have significant positive effects on U.S. waterways by consuming Asian carp. With experience gained by the U.S. Fish and Wildlife Service from past successful reintroductions of animals such as gray wolves and California condors, the department is definitely capable of succeeding at yet one more species reintroduction, this time with alligator gar, and these efforts are already underway in Illinois (Department of Natural Resources, n.d., para. 5).

         Asian carp continue to spread and cause problems for the native fish wherever they invade. In these areas, native fish populations decrease by about half through being outcompeted themselves or through their food sources dying off (Phelps et al., 2017, p.6). With Asian carp threatening to establish themselves in the great lakes, a billion dollar per year fishing industry comes under fire, as well as an amazing and unique ecosystem. Nature controls populations through a system of checks and balances. This means that if something were to keep carp populations in check, they wouldn’t be such a big problem. For example reintroducing wolves to the Yellowstone national park region to control the effects of the elk population that was getting way out of control worked out very well and allowed the degraded conditions of the willow trees that the elk feed on to increase immensely (“1995 Reintroduction of wolves in Yellowstone,” 2017, “Wolf reintroduction changes ecosystem,” 2011). In a similar way alligator gars are the answer to keeping the effects of the Asian carp in check. They grow large enough to eat them and other gars have shown a taste for asian carp. They also were native to the region before, so reintroducing them is not some radical, new idea. If you have a large prey item, introduce a larger predator to keep it in check, and that is exactly what we propose to do with the alligator gar in regards to the Asian carp epidemic that threatens the Mississippi River Valley Basin and great lakes.


Samuel Romania – Environmental Science Major

Jonathan Hastings – NRC:Fisheries

Skyler Rehbein – NRC Fisheries



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